The experimental approach aims to treat disease by replacing a broken gene with a functioning one. Unlike drugs that must be taken for months or years, gene therapy offers the tantalizing promise of a one-time treatment — and biotech startups are racing to get the first federal approvals to put gene therapies on the market.

Spark Therapeutics, a two-year-old company based in Philadelphia, has set a target date of next year to launch its experimental gene therapy for rare, inherited disorders that can lead to blindness.

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Bluebird Bio, a startup based in Cambridge, Mass., is also in the hunt. Chief executive Nick Leschly said he believes the time is finally right to bring a gene therapy treatment to market after decades of widespread disappointment in the field, including a high-profile patient death back in 1999 and hundreds of clinical trials that went nowhere.

The company believes “that the next five years are the five years that gene therapy has been looking for for the last 35 years,” Leschly told investors during a presentation on Tuesday at the J.P. Morgan Healthcare Conference.

That optimism was a bit discordant following a rough few weeks for gene therapy.

Bluebird’s stock plummeted in the second half of last year, a slide made worse last month when the company announced mixed results from its trials testing gene therapies for sickle-cell disease and the blood disorder beta thalassemia.

Also in recent weeks, the Dutch gene therapy company UniQure canned its plan to seek approval in the United States for a gene therapy approved in Europe to treat a rare disorder that can cause pancreatitis. The company cited reimbursement challenges and a tiny patient market.

And this week the Maryland biotech company GenVec’s stock was sent spiraling down when enrollment of new patients in an early-stage trial testing its experimental gene therapy for hearing loss was halted. The reason: an independent monitoring board recommended a safety check.

To be sure, the news wasn’t entirely glum: Last week UniQure reported encouraging results from a small, early-stage study of patients with hemophilia.

Still, the recent stumbles underscore the fact that gene therapy remains a very tough challenge. Patients seem to have wildly different responses to gene therapies over time. It’s hard to deliver an edited gene to all the body parts that need it.

And then there’s the worry about pricing. One-time gene therapy treatments are expected to command sky-high prices, further burdening a health care system already struggling to bear the rising costs of drugs.

When asked about the company’s pricing plans at Spark’s Q&A session with investors here on Monday, chief executive Jeff Marrazzo joked that he’d “never heard that question before.” Marrazzo didn’t disclose anything notable in that conversation, but in the past he has publicly floated the prospect of alternative pricing models, such as “pay-per-performance” plans in which the company would get full reimbursement only if its treatment helped a patient.

Despite those challenges, industry titans like Pfizer, Novartis, and Shire have recently upped their investments in gene therapy, betting that the tricky technology will be worth the payoff.

That shift in attitude was articulated here by Sven Kili, head of gene therapy development at GlaxoSmithKline. He said during a panel discussion on Monday that although GSK had been a “little bit of a sleeper” when it came to developing gene and other cell therapies in recent years, “just recently we’ve started coming out.”

“GSK as a company has really decided to look at this area and invest very substantially not only in people but also in technology and the ability to move these products forward,” Kili said.